This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Field modulation systems currently employed in the ACERT c.w. HF spectrometers consist of an suitably-modified audio power amplifier driving a field-generating coil. Because the coil represents a highly reactive load, the field modulation network operates most efficiently and at highest peak field value when series-tuned to resonance. Historically, tuning of the field modulation systems has entailed the insertion an intermediary network for switching one of a stepped-value series of capacitors into the modulation circuit while observing with an oscilloscope the resulting coil voltage amplitude. This adjustment is made with the goal of maximizing the coil voltage for a given constant amplifier output signal at the chosen excitation frequency. However, the capacitor values available in the existing networks through variation of the switch setting are only coarsely related due to the large capacitance range required and finite number of switch positions. Therefore, a selected capacitor value that results in maximum amplitude may, in actuality, be considerably displaced from the value required for attaining the true resonance peak. With design and construction of a replacement tuning unit which we have designated the (high-field) Modulation Fine-Tuning Network (HF-MFTN), we have addressed the historical problem of inadequate field-modulation tuning resolution and at the same time added both built-in monitoring capability and galvanically-isolated outputs for optional external instrumentation (e.g., oscilloscope, frequency counter). We have recently constructed 3 HF-MFTN units. After debug, test and calibration, two of these devices will be used at ACERT and the remaining device will be delivered to SUNY Albany. Of the ACERT units, one network is to be installed in our 170/240 GHz spectrometer and the other in our 95 GHz c.w./pulse spectrometer.